This invention relates to a hydrogen-absorbing alloy, to an electrode containing the hydrogen-absorbing alloy, and to a secondary battery comprising a negative electrode comprising the hydrogen-absorbing alloy.
Since hydrogen-absorbing alloy is capable of safely and easily storing hydrogen as an energy source, hydrogen-absorbing alloy is now attracting many attentions as a new energy exchange material or a new energy storage material. Thus, there have been proposed various applications of hydrogen-absorbing alloy as a new functional raw material, such as the storage and transport of hydrogen, the storage and transport of heat, the conversion of heat energy to mechanical energy, the separation and purification of hydrogen, the separation of hydrogen isotope, a battery employing hydrogen as an active material, a catalyst in synthetic chemistry, and a temperature sensor.
Recently, a nickel-hydrogen secondary battery where a hydrogen-absorbing alloy is employed as a negative electrode material has been attracting many attentions as a public-use battery of next generation, because the battery is advantageous in various aspects, e.g. it is high in capacity, highly resistive to overcharging and overdischarging, capable of performing a high rate charge/discharge, free from environmental pollution, and interchangeable with a nickel-cadmium battery. Accordingly, many attempts have been intensively made at present for the application and actual use of the nickel-hydrogen battery.
As evident from these facts, the hydrogen-absorbing alloy has many possibilities for various applications in view of its useful physical and chemical characteristics, so that the hydrogen-absorbing alloy is now considered as being one of important raw materials in future industries.
The metal capable of absorbing hydrogen may be a metal element which reacts exothermically with hydrogen, i.e. which is capable of forming a stable compound together with hydrogen (for example, Pd, Ti, Zr, V, a rare earth element or an alkaline earth element); or an alloy comprising an above-mentioned metal element.
One of the advantages of employing an alloy is that the bonding strength between a metal and hydrogen can be suitably weakened so that not only the absorption reaction but also the desorption reaction can be performed comparatively easily. Second advantage of employing an alloy is that the absorption and desorption characteristics of the alloy such as the magnitude of hydrogen gas pressure required for the reaction (equilibrium pressure; plateau pressure), the extent of equilibrium region (plateau region), or the change of equilibrium pressure during the process of absorbing hydrogen (flatness), etc. can be improved. Third advantage of employing an alloy is the improvement in chemical and physical stability of the metal capable of absorbing hydrogen.
The composition of the conventional hydrogen-absorbing alloy may be classified into the following types;
(1) a rare earth element type (for example, LaNi.sub.5, MmNi.sub.5, etc.); PA1 (2) a Laves type (for example, ZrV.sub.2, ZrMn.sub.2, etc.); PA1 (3) a titanium type (for example, TiNi, TiFe, etc.); PA1 (4) a magnesium type (for example, Mg.sub.2 Ni, MgNi.sub.2, etc.); and PA1 (5) other types (for example, cluster, etc.). PA1 wherein A is at least one kind of element which is capable of generating heat of formation .DELTA.H (kJ/mol) of less than 20 kJ/mol at the occasion of generating a hydride from one mole of hydrogen at a temperature of 25.degree. C.; B is at least one kind of element which is capable of generating heat of formation .DELTA.H (kJ/mol) of not less than 20 kJ/mol at the occasion of generating a hydride from one mole of hydrogen at a temperature of 25.degree. C.; and X is a ratio in number of the aforementioned at least one A.sub.2 B.sub.4 subcell to the aforementioned at least one AB.sub.5 subcell. PA1 wherein A is at least one kind of element which is capable of generating heat of formation .DELTA.H (kJ/mol) of less than 20 kJ/mol at the occasion of generating a hydride from one mole of hydrogen at a temperature of 25.degree. C.; B is at least one kind of element which is capable of generating heat of formation .DELTA.H (kJ/mol) of not less than 20 kJ/mol at the occasion of generating a hydride from one mole of hydrogen at a temperature of 25.degree. C.; and X is a ratio in number of the aforementioned at least one A.sub.2 B.sub.4 subcell to the aforementioned at least one AB.sub.5 subcell. PA1 wherein A is at least one kind of element which is capable of generating heat of formation .DELTA.H (kJ/mol) of less than 20 kJ/mol at the occasion of generating a hydride from one mole of hydrogen at a temperature of 25.degree. C.; B is at least one kind of element which is capable of generating heat of formation .DELTA.H (kJ/mol) of not less than 20 kJ/mol at the occasion of generating a hydride from one mole of hydrogen at a temperature of 25.degree. C.; and X is a ratio in number of the aforementioned at least one A.sub.2 B.sub.4 subcell to the aforementioned at least one AB.sub.5 subcell.
By the way, the rare earth element-Ni based intermetallic compound represented by the aforementioned type (1) includes many number of compounds other than an AB.sub.5 type compound. For example, Mat. Res. Bull., 11, (1976) 1241 describes that an intermetallic compound containing a larger quantity of rare earth element as compared with the AB.sub.5 type compound is capable of absorbing a larger quantity of hydrogen in the vicinity of normal temperature as compared with the AB.sub.5 type compound. It is also reported that a magnesium-rare earth element based alloy, which is a magnesium-substituted rare earth-Ni based alloy, is capable of absorbing a large quantity of hydrogen gas (Y. Ohsumi, "Soda and Chlorine", 34, 447 (1983)).
It is pointed out by H. Oesterreicher et al in J. Lee-Common Met, 73,339 (1980) that La.sub.1-X Mg.sub.X Ni.sub.2 type alloys for example among the alloys having such compositions are accompanied with a problem that the hydrogen-releasing rate thereof is very low due to the high stability thereof to hydrogen.
There is also a report on a PuNi.sub.3 type hydrogen-absorbing alloy having a composition of Mg.sub.2 LaNig, which was published by K. Kadir et al in the summary of lecture in the 120th Spring Meeting of Japan Metallic Society, p. 289 (1997).